This invention relates to a method for controlling an injection molding machine for smoothly and accurately bringing an injection nozzle of an injection apparatus into contact with an opening of a mold of a molding apparatus through which material resin is injected to obtain mold products.
In designing an injection molding machine particularly by taking into consideration the automatic operations or controls of the various steps, attentions should be paid to the improvement of quality of products, the energy-saving and the improvement of productivity, and these attentions should be paid particularly to the material resin measuring process. Regarding a point of the improvement of the quality of the products, since the quality is mainly affected by the injection speed, pressure, resin temperature, injection amount and the like, it is required to measure a precise amount of the resin to be injected with high precision. Regarding a point of the energy-saving, the material resin in particle form fed into a heating cylinder of an injection molding machine is heated, sheared and kneaded therein by a screw to obtain an evenly molten resin and the energy required for the resin measuring stroke is minimized by effectively controlling the operation of the screw in conformity with the rate of revolution (called revolution number hereinafter), the back pressure of the screw, the kind or type of the resin and the temperature of the resin to be melted. Moreover, the productivity of the mold products can be achieved by minimizing the time interval required for the measuring process.
Generally, in an injection molding machine, material resin is fed into a heating cylinder through a hopper and the resin fed into the heating cylinder is fed forwardly by rotating a screw. During this operation, the resin is heated by a heater such as heating coil located on the outer periphery of the heating cylinder and then sheared and kneaded by rotating the screw. When the resin fed into a space formed at the front end portion of the heating cylinder is injected into a mold through a nozzle formed at the front end of the heating cylinder, the screw is then slightly retracted by the pressure of the resin injected into the mold, thereby preventing the molten resin from flowing outwardly. At this time, a back pressure is applied to the screw by a drive mechanism operatively connected to the screw, thereby preventing the introduction of air into the heating cylinder through the nozzle and the hopper, and measuring the resin amount precisely. Accordingly, the screw is gradually retracted by the pressure difference between the resin pressure and the back pressure applied by the drive mechanism. The revolution number and the back pressure of the screw are preset experientially by the kind and temperature of the resin to be used, and the position of the screw in the cylinder for determining the resin amount for injection is set by a detecting means such as a limit switch, which is disposed in the drive mechanism, as well as means for rotating the screw and means for applying the back pressure to the screw.
With the construction of the injection molding machine of conventional type described above, the position of the screw is detected by the limit switch and accordingly controlled indirectly by the relative relationships between the revolution number of the screw and the back pressure thereof. This involves such a problem that the screw may stop at a position beyond the desired stop position for the reason that the rotation of the screw is stopped after the operation of the limit switch. In order to obviate this defect in the prior technique there is no countermeasure other than to gradually slow down the rotation of the screw as the screw approaches the limit switch, or to set the operating point of the limit switch before the theoretical operating point by taking into consideration the excessive backward movement of the screw. For the reason described above, it is necessary in actual to determine the screw position during the resin measuring stroke by repeating the trials and errors. Furthermore, it is difficult to precisely set the operating point of the limit switch based on external factors such as kinds of resins to be used, shapes of molds, temperature variation, moisture of the resin and the variation of the resin amount fed from the hopper into the heating cylinder, thus being difficult to obtain the accurate resin amount for injection as well as to set an accurate operating point of the limit switch. It is, therefore, impossible to evenly inject the resin into the mold and to prevent the degradation of the quality of the molded products. Moreover, in this conventional technique, it is obliged to unnecessarily reduce the revolution numbers of the screw even in the normal operation to stop the screw at the accurate position, thus measurement of the resin amount in a short time, high energy efficiency as well as the improvement of the rate of production being required.
In addition to these problems, when it is required to bring the nozzle portion into contact with the mold opening in a prior art technique, the nozzle portion, i.e. the heating cylinder, is moved towards the mold under a speed control which is then switched to a pressure control when the nozzle portion nearly approaches to the mold. However, it is considerably difficult to know at which position and at what time the speed control is to be switched to the pressure control, and moreover, on this switching time, the injection speed or pressure is adversely changed discontinuously. In the prior art technique, there are found out substantially the same problems in the mold clamping operation and the resin injection operation.